The present invention relates to audio cables in general and in particular to a method and apparatus comprising a passive network particularly suitable for use in cables designed to connect electrical musical instruments to amplifiers.
Musicians of electrical instruments have long been aware that the audio qualities of their music depend in part on the total system comprising the instrument, the cable and the amplifier. Generally, within the range of audio frequencies produced by a particular electrical instrument, cable and amplifier system, there is a smaller range in which the sounds produced are xe2x80x9cemphasizedxe2x80x9d or xe2x80x9cnoisy.xe2x80x9d The actual range depends on the length, materials and construction of the cable.
Graphic equalizers, which utilize active filters, may be used to compensate for this effect. Usually the compensation requires an amplification of the sounds at the lower and higher end of the relevant frequency range, with a gradual decrease in the amplification for each frequency range toward the center frequency which produces the familiar xe2x80x9cVxe2x80x9d on the graphic equalizer controls. Active filter compensation, however, can produce its own noise. Furthermore, a more evenly balanced signal expands the control choices afforded to the musician by a graphic equalizer. Because of the required compensation, such choices are usually limited in current standard systems.
The inventors of the present invention discovered that there was a relationship between the rate of change in the phase angle of the cable and the perceived clarity and xe2x80x9cevennessxe2x80x9d of the music produced by the system. Where the rate of change in the phase angle was at a maximum the sound produced was the most xe2x80x9cemphasizedxe2x80x9d or xe2x80x9cnoisy.xe2x80x9d The relationship could be quantified in terms of the frequency at which the positive conductor of the cable had a phase angle of 45xc2x0 (or, alternatively, as the frequency where the quality factor (Q) was equal to 1) as a result of the inherent inductance of the cable.
The inventors discovered that by selectively connecting passive RC or RLC circuits between the positive and negative (or ground) conductors of the cable, where the RC or RLC circuits had a phase angle of xe2x88x9245xc2x0 at selected frequencies that were within the audio frequency range, approximately equally distributed on a logarithmic scale, and not equal to the frequency where the cable naturally had a phase angle of 45xc2x0, the resulting audio output could be modified to significantly reduce the xe2x80x9cemphasizedxe2x80x9d or xe2x80x9cnoisyxe2x80x9d effect, and produce a more uniform audio output over the range of frequencies produced by the system.
In view of the foregoing, principal object of the present invention is a method and apparatus comprising a passive network which can be incorporated into a coaxial or any other type of cable used to connect electrical musical instruments to amplifiers, that modifies the overall audio effect produced by the system comprising the instrument or source of the audio signal, the cable and the amplifier.
In one embodiment of the present invention, the network comprises a plurality of RC circuits coupled in parallel, connected between the positive conductor and ground of a cable suitable for connecting an electrical musical instrument to an amplifier. There is a frequency at which the positive conductor""s natural phase angle, produced as a result of inherent inductance, is 45xc2x0. Each of the circuits is selected to have a phase angle of xe2x88x9245xc2x0 at a different frequency, which frequency is in the audio frequency range between approximately 20 Hz and 4 kHz and above and/or below the frequency at which the cable has a phase angle of 45xc2x0. The particular frequencies selected depend upon the range of audible frequencies produced by the instrument or other source of audio signal and the desired effect(s). The component values of the circuits are selected such that the measured impedance of the network at all frequencies in the relevant frequency range (i.e., the audible frequencies for the system) is equal to or greater than 0.20 Mxcexa9 and the overall combined capacitance of the cable and network is such that the xe2x80x9croll-offxe2x80x9d in the relevant frequency range is not large enough to have an audible effect, which generally means that the capacitance of the circuits must be very low.
In some alternative embodiments, the plurality of circuits comprise RLC circuits where the components are selected using similar criteria. In other embodiments the network can be designed to emphasize certain frequencies.